Dendritic cells can prime anti-tumor CD8⁺ T cell responses through major histocompatibility complex cross-dressing

Highlights

• Optimal anti-tumor CD8⁺ T cell priming requires MHC-I expression on cancer cells

• Tumor-resident DCs and macrophages dress with cancer cell MHC-I in vivo

• MHC-I cross-dressing is sufficient for antigen-specific CD8⁺ T cell priming ex vivo

• The impact of MHC-I cross-dressing on CD8⁺ T cell priming differs between tumor models

Summary

Antigen cross-presentation, wherein dendritic cells (DCs) present exogenous antigen on major histocompatibility class I (MHC-I) molecules, is considered the primary mechanism by which DCs initiate tumor-specific CD8⁺ T cell responses. Here, we demonstrate that MHC-I cross-dressing, an antigen presentation pathway in which DCs acquire and display intact tumor-derived peptide:MHC-I molecules, is also important in orchestrating anti-tumor immunity. Cancer cell MHC-I expression was required for optimal CD8⁺ T cell activation in two subcutaneous tumor models. In vivo acquisition of tumor-derived peptide:MHC-I molecules by DCs was sufficient to induce antigen-specific CD8⁺ T cell priming. Transfer of tumor-derived human leukocyte antigen (HLA) molecules to myeloid cells was detected in vitro and in human tumor xenografts. In conclusion, MHC-I cross-dressing is crucial for anti-tumor CD8⁺ T cell priming by DCs. In addition to quantitatively enhancing tumor antigen presentation, MHC cross-dressing might also enable DCs to more faithfully and efficiently mirror the cancer cell peptidome.

Introduction

The anti-tumor immune response has been intensely studied over the past several decades (Vesely et al., 2011), leading to the development and widespread administration of cancer immunotherapies in the clinic (Baumeister et al., 2016; Waldman et al., 2020) However, cancer immunotherapy is not universally effective (Dougan et al., 2019), and progress is hindered by persisting gaps in our understanding of the fundamental mechanisms required for the orchestration of effective anti-cancer immune responses, particularly at the level of early T cell priming. Although the mechanisms by which CD8⁺ T cells are primed against human cancers remain unclear, murine models have provided critical insights into these questions. Recently, the professional antigen presenting cells (APCs) involved in anti-tumor T cell priming have been identified (Roberts et al., 2016; Salmon et al., 2016); however, the specific pathways utilized by these APCs to acquire and present tumor antigens in vivo remain incompletely defined.

Migratory CD103⁺ and CD11b⁺ dendritic cells (DCs) exist in tissues throughout the body, constitutively acquiring proteins from surrounding cells before trafficking to draining lymph nodes, where they present derivative peptide antigens to T cells in the context of major histocompatibility complex (MHC) molecules (Merad et al., 2013; Worbs et al., 2017). Migratory DCs within tumors similarly acquire and subsequently present cancer cell-derived antigens to T cells in tumor-draining lymph nodes (tdLNs) (Wculek et al., 2020). Recent evidence indicates that tumor antigens are exclusively transported to tdLNs by migratory CD103⁺ DCs (Ruhland et al., 2020; Salmon et al., 2016) and that these DCs are primarily responsible for anti-tumor CD8⁺ T cell priming in vivo (Roberts et al., 2016; Salmon et al., 2016). Migratory CD103⁺ DCs, along with lymph node resident CD8α⁺ DCs, comprise the BATF3- and IRF8-dependent conventional type 1 DC (cDC1) lineage (Anderson et al., 2021; Edelson et al., 2010; Hildner et al., 2008). Because antigen cross-presentation is a canonical function of cDC1 (Dudziak et al., 2007; Hildner et al., 2008) and because numerous studies have demonstrated a requirement for cDC1 in the activation of anti-tumor CD8⁺ T cell responses (Fuertes et al., 2011; Hildner et al., 2008; Wculek et al., 2020), prevailing thought is that tumor-specific CD8⁺ T cells are primed exclusively through antigen cross-presentation (Lee et al., 2020; Sánchez-Paulete et al., 2017).

However, an alternative antigen presentation mechanism has been described, in which DCs acquire and present intact peptide:MHC (pMHC) captured directly from neighboring cells (André et al., 2004; Dolan et al., 2006; Herrera et al., 2004; Russo et al., 2000; Wolfers et al., 2001). This phenomenon, known as MHC cross-dressing, has been implicated in antigen presentation in various contexts ranging from viral infection (Wakim and Bevan, 2011) and vaccination (Li et al., 2012) to thymic selection (Koble and Kyewski, 2009; Kroger et al., 2017; Perry et al., 2018), graft rejection (Liu et al., 2016), and peripheral tolerance to maternal microchimerism (Bracamonte-Baran et al., 2017). By necessity, conclusions regarding the role of MHC cross-dressing in antigen presentation by DCs have largely arisen from in vitro studies and in vivo models of MHC-mismatched bone marrow chimeric mice (Dolan et al., 2006; Koble and Kyewski, 2009; Wakim and Bevan, 2011) or solid organ transplantation (Liu et al., 2016), due to difficulty in controlling for cross-presentation without entirely ablating antigen presentation on MHC-I. Indeed, although MHC cross-dressing by DCs is sufficient to induce T cell priming in various contexts, the necessity of this antigen presentation pathway in mediating in vivo T cell activation has never been conclusively demonstrated in syngeneic hosts.

In this study, we employed two syngeneic murine tumor models expressing distinct model antigens presented in the context of the MHC-I molecule, H-2K^b (K^b), to determine the extent to which MHC-I cross-dressing was involved in antigen-specific CD8⁺ T cell priming. Although the magnitude of the effect varied, CD8⁺ T cell priming against K^b-restricted tumor antigens was impaired in mice harboring K^b−/− tumors, despite the cross-presentation pathway being fully intact. Furthermore, cancer cell-derived MHC-I molecules were readily observed within and on the surface of tumor-resident APCs, and MHC-I-deficient CD103⁺ cDC1 isolated from tdLNs of mice bearing K^b-sufficient tumors stimulated antigen-specific CD8⁺ T cells ex vivo. The importance of WDFY4-dependent antigen cross-presentation (Theisen et al., 2018) in mediating anti-tumor CD8⁺ T cell responses differed across experimental models, but in some cases, it was dispensable for in vivo CD8⁺ T cell priming. Finally, we observed that APCs became cross-dressed with human leukocyte antigen class I (HLA-I) molecules upon co-culture with HLA-mismatched tumor cells and in tumors xenografted in immunodeficient mice. Acquisition of tumor cell-derived HLA-I molecules correlated with uptake of tumor antigens in vitro. Similarly, APCs isolated from murine tumors acquired both tumor-derived MHC-I and fluorescent antigen in vivo, suggesting that MHC-I cross-dressing and internalization of tumor material may be linked processes. Taken together, our results demonstrate that MHC-I cross-dressing is central to the ability of cDC1 to orchestrate anti-tumor CD8⁺ T cell responses.